There are ongoing efforts to provide next generation mobile communications and to improve existing communications among networked devices including among wireless devices. A recent development has to been not only to do away wires or cables within a building or home but also to do away with the cables coming into the building or home. WiMAX technology (also known as 802.16 because it is based on the IEEE 802.16 WirelessMAN Standard for Wireless Metropolitan Area Networks) promises to allow this jump to more wireless communications that function as a wireless alternative to cable modems and DSL (Digital Subscriber Line). WiMAX will likely offer connectivity at up to 30 miles from an antenna at speeds up to 75 mbps (megabytes per second) and at higher rates or speeds under 5 miles, whereas a cable modem may only offer speeds of 1 mbps. As a result, where cable and telephone companies do not offer broadband Internet connections, WiMAX technology offers a way to provide broadband Internet, digital TV, and other digital communications with the use of a wireless antenna to pick up a WiMAX signal that is then distributed wireless (or with wires) throughout the local area by a base station (BS) to user terminals or devices (e.g., to subscriber stations (SSs)).
WiMAX supports a metropolitan area network (MAN) rather than a local area network (LAN) and assumes a point-to-multipoint topology. A controlling base station (BS) connects subscriber stations (SSs) not to each other but to various public networks that are linked to the base station. In this wireless communication system, a remote subscriber station (SS), such as a cellular or mobile telephone, accesses a network by sending an access signal to a base station (BS) or “uplinks.” The access signal fulfills important functions such as requesting resource allocation from the BS, alerting the BS of the existence of the SS that is trying to enter the network, and initiating a process that allows the BS to measure some parameters of the SS (e.g., timing offset caused by propagation, frequency error, transmit power, and the like) that are maintained and adjusted to ensure a non-interfering sharing of the uplink resource (e.g., the BS). Unlike ordinary data traffic that is sent using scheduled resources that are allocated to the SS, the access signal for the SS is transmitted in an unsolicited manner and this process is often referred to as random access. This process may also be thought of as ranging because the access signal can help the BS to measure the propagation distance from the SS (i.e., its range) so that its transmission timing can be adjusted to ensure the signals from all the SSs are synchronized at the BS (i.e., uplink timing synchronization). Synchronization of the numerous, multi-user signals from the SSs has presented a number of problems that will be discussed after briefly describing transmission techniques or protocols.
OFDM (Orthogonal Frequency Division Multiplexing) technique has been widely proposed in many wireless systems to provide high data rate transmission. OFDM uses a set of overlapping but orthogonal sub-carriers to realize high spectrum efficiency. More recently, combined with TDMA (Time Division Multiple Access) and/or FDMA (Frequency Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access) has been proposed in many broadband wireless systems such as WiMAX systems. OFDMA can be divided into 2 types: sub-band OFDMA and interleaved OFDMA. In sub-band OFDMA, a set of consecutive sub-carriers along a frequency axis are allocated to a certain user so that signals from different users can be separated by filter banks at the receiver or BS side. Obviously, this type of OFDMA is sensitive to frequency-selective channel. To take advantage of channel diversity gain, interleaved OFDMA allocates a certain user a set of interleaved sub-carriers that allow a large sub-carrier spacing for each user making interleaved OFDMA more desirable for WiMAX.
However, synchronization issues may arise between a user device or subscriber station (SS) and a particular base station (BS) or its uplink receiver. For example, due to one or more of misalignments from an SS to a BS, discordance between the BS oscillator and an SS's oscillator(s), and Doppler effects, synchronization errors related to time delays and frequency offsets are often present in uplink. As with the OFDM technique, OFDMA is so vulnerable or susceptible to these synchronization errors that even a small frequency offset results in loss of orthogonality, and in OFDMA, time delays in the time domain often lead to complex exponential twiddles or variations in the frequency domain. Additionally, in contrast to downlink (or forward link), the received signals in uplink are subject to multi-user time delays and frequency offsets as multiple points or devices communicate with a single base station. Moreover, interleaved OFDMA complicates performing effective signal synchronization due to the interleaved sub-carriers. For instance, if two consecutive sub-carriers are allocated to two users, they may be subject to two different time delays and frequency offsets. The inter-channel-interference (ICI) between the two sub-carriers typically results in Multiple Access Interference (MAI) between the two users which can cause problems during synchronization.
More specifically, under one mandatory allocation scheme, a tile-based interleaved OFDMA is used in WiMAX communication system (e.g., as defined in IEEE802.16d/e Uplink). In this scheme, every four consecutive sub-carriers are grouped along a frequency axis, and this group is denoted as a tile. These tiles are interleaved in a given permutation base. Then, the interleaved tiles are regrouped into sub-channels by which radio resources are allocated to the users or SSs by the BS. In this manner, the data from one user are uniformly distributed onto the overall band. Besides, in order to take advantage of time diversity, the assignment scheme is rotated during transmission time. Tile-based interleaved OFDMA makes use of a ranging procedure to detect and adjust users' transmission parameters. Ranging can be divided into two types: initial ranging and periodic ranging. These ranging operations are completed interactively by the base station and users. Through the ranging procedure (initial and periodic), the time delay and frequency offset of an active user could be adjusted within an acceptable criterion, and a base-station could obtain a relatively accurate estimation of the residual time delays and frequency offsets of the active users. In this ranging, use of a loose criterion brings a relative long time delay and large frequency offset but needs less ranging signaling. In contrast, use of a strict criterion produces a short time delay and narrow frequency offset but needs more ranging signaling. Thus, in designing a robust base station receiver or uplink receiver, a significant problem that must be addressed is how to effectively and efficiently cancel the MAI (or correct for MAI) due to multi-user residual time delays and frequency offsets that can detrimentally effect synchronization at the base station.
A number of techniques have been suggested to address issues with MAI, but each has created new difficulties or has not adequately addressed the needs of the wireless communication industry. One suggested solution is applied to interleaved OFDMA uplink synchronization and involves the use of a banded interference matrix. The matrix is constructed with a priori knowledge of the frequency offsets of the users, and a correction matrix is generated based on least square (LS) algorithm. One insufficiency with this proposed solution is that it does not take into account time delays in constructing the banded interference matrix. Another disadvantage is that its high computational complexity makes it difficult to be implemented on an OFDMA system with a large number of available sub-carriers. This is important in WiMAX systems such as those defined by IEEE 802.16d/e as the assignment of the available sub-carriers to the users is frequently updated and a high complex computation of a correction matrix becomes inapplicable to real-time transmission.
In another proposed solution, SIC (Successive Interference Cancellation) and SPIC (Selective Parallel Interference Cancellation) algorithms are applied during OFDMA uplink synchronization. Under this proposal, even if there is no need to inverse a matrix, the algorithms are iterated with a channel estimator and demodulator. Due to the iterative feature, this proposal has a high complexity, which is so difficult to estimate that a stable latency implementation cannot be readily realized. Further, in this proposed solution, the self-distortions caused by time delay and frequency offset are unacceptably ignored. In another proposed solution, reference sub-carriers are allocated to every active user for synchronization in uplink. Code Division Multiple Access (CDMA) codes are transmitted on these reference sub-carriers. The base station receiver detects time delays and frequency offsets through the reference sub-carriers and requires the active users to adjust their transmission parameters through downlink until the active users are synchronized with base-station. This procedure is similar to the aforementioned ranging procedure. Unfortunately, it needs a long-time signaling to do synchronizations for each user. Besides, in a mobile system, the varying Doppler frequencies are difficult to adjust out making this proposal undesirable. Another proposed method allows transmitting upper and lower edge side-lobe canceling signals over respective sub-carriers nearest to sub-bands (including guard interval). At the receiver side, after inverse fast Fourier transform (IFFT) transformation, the canceling signals can reduce the MAI. This solution defines the algorithm of generating the upper and lower side-lobe canceling signals and the way to insert them into guard intervals, but a problem with this method is that it needs to change the signal transmission structure, which makes the method incompatible with standardized wireless communications (such as those defined by IEEE 802.16 or the like).
Hence, there remains a need for an improved uplink method and uplink receivers and base stations that incorporate such as method that address issues with MAI between multiple users or SSs that can prevent accurate synchronization at the uplink receiver or base station (BS). Preferably, such a method is designed to provide both time and frequency synchronization at an uplink receiver (e.g., an OFDMA uplink receiver) while having a low complexity and an acceptably small memory requirement.